Re: [PATCH v9 6/8] KVM: Update lpage info when private/shared memory are mixed

Chao Peng
2022-11-09T04:18:50Z

On Tue, Nov 08, 2022 at 08:08:05PM +0800, Yuan Yao wrote:
> On Tue, Oct 25, 2022 at 11:13:42PM +0800, Chao Peng wrote:
> > When private/shared memory are mixed in a large page, the lpage_info may
> > not be accurate and should be updated with this mixed info. A large page
> > has mixed pages can't be really mapped as large page since its
> > private/shared pages are from different physical memory.
> >
> > Update lpage_info when private/shared memory attribute is changed. If
> > both private and shared pages are within a large page region, it can't
> > be mapped as large page. It's a bit challenge to track the mixed
> > info in a 'count' like variable, this patch instead reserves a bit in
> > 'disallow_lpage' to indicate a large page has mixed private/share pages.
> >
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > ---
> > arch/x86/include/asm/kvm_host.h | 8 +++
> > arch/x86/kvm/mmu/mmu.c | 112 +++++++++++++++++++++++++++++++-
> > arch/x86/kvm/x86.c | 2 +
> > include/linux/kvm_host.h | 19 ++++++
> > virt/kvm/kvm_main.c | 16 +++--
> > 5 files changed, 152 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 7551b6f9c31c..db811a54e3fd 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -37,6 +37,7 @@
> > #include <asm/hyperv-tlfs.h>
> >
> > #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
> > +#define __KVM_HAVE_ARCH_UPDATE_MEM_ATTR
> >
> > #define KVM_MAX_VCPUS 1024
> >
> > @@ -952,6 +953,13 @@ struct kvm_vcpu_arch {
> > #endif
> > };
> >
> > +/*
> > + * Use a bit in disallow_lpage to indicate private/shared pages mixed at the
> > + * level. The remaining bits are used as a reference count.
> > + */
> > +#define KVM_LPAGE_PRIVATE_SHARED_MIXED (1U << 31)
> > +#define KVM_LPAGE_COUNT_MAX ((1U << 31) - 1)
> > +
> > struct kvm_lpage_info {
> > int disallow_lpage;
> > };
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 33b1aec44fb8..67a9823a8c35 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -762,11 +762,16 @@ static void update_gfn_disallow_lpage_count(const struct kvm_memory_slot *slot,
> > {
> > struct kvm_lpage_info *linfo;
> > int i;
> > + int disallow_count;
> >
> > for (i = PG_LEVEL_2M; i <= KVM_MAX_HUGEPAGE_LEVEL; ++i) {
> > linfo = lpage_info_slot(gfn, slot, i);
> > +
> > + disallow_count = linfo->disallow_lpage & KVM_LPAGE_COUNT_MAX;
> > + WARN_ON(disallow_count + count < 0 ||
> > + disallow_count > KVM_LPAGE_COUNT_MAX - count);
> > +
> > linfo->disallow_lpage += count;
> > - WARN_ON(linfo->disallow_lpage < 0);
> > }
> > }
> >
> > @@ -6910,3 +6915,108 @@ void kvm_mmu_pre_destroy_vm(struct kvm *kvm)
> > if (kvm->arch.nx_lpage_recovery_thread)
> > kthread_stop(kvm->arch.nx_lpage_recovery_thread);
> > }
> > +
> > +static inline bool linfo_is_mixed(struct kvm_lpage_info *linfo)
> > +{
> > + return linfo->disallow_lpage & KVM_LPAGE_PRIVATE_SHARED_MIXED;
> > +}
> > +
> > +static inline void linfo_update_mixed(struct kvm_lpage_info *linfo, bool mixed)
> > +{
> > + if (mixed)
> > + linfo->disallow_lpage |= KVM_LPAGE_PRIVATE_SHARED_MIXED;
> > + else
> > + linfo->disallow_lpage &= ~KVM_LPAGE_PRIVATE_SHARED_MIXED;
> > +}
> > +
> > +static bool mem_attr_is_mixed_2m(struct kvm *kvm, unsigned int attr,
> > + gfn_t start, gfn_t end)
> > +{
> > + XA_STATE(xas, &kvm->mem_attr_array, start);
> > + gfn_t gfn = start;
> > + void *entry;
> > + bool shared = attr == KVM_MEM_ATTR_SHARED;
> > + bool mixed = false;
> > +
> > + rcu_read_lock();
> > + entry = xas_load(&xas);
> > + while (gfn < end) {
> > + if (xas_retry(&xas, entry))
> > + continue;
> > +
> > + KVM_BUG_ON(gfn != xas.xa_index, kvm);
> > +
> > + if ((entry && !shared) || (!entry && shared)) {
> > + mixed = true;
> > + goto out;
> > + }
> > +
> > + entry = xas_next(&xas);
> > + gfn++;
> > + }
> > +out:
> > + rcu_read_unlock();
> > + return mixed;
> > +}
> > +
> > +static bool mem_attr_is_mixed(struct kvm *kvm, struct kvm_memory_slot *slot,
> > + int level, unsigned int attr,
> > + gfn_t start, gfn_t end)
> > +{
> > + unsigned long gfn;
> > + void *entry;
> > +
> > + if (level == PG_LEVEL_2M)
> > + return mem_attr_is_mixed_2m(kvm, attr, start, end);
> > +
> > + entry = xa_load(&kvm->mem_attr_array, start);
> > + for (gfn = start; gfn < end; gfn += KVM_PAGES_PER_HPAGE(level - 1)) {
> > + if (linfo_is_mixed(lpage_info_slot(gfn, slot, level - 1)))
> > + return true;
> > + if (xa_load(&kvm->mem_attr_array, gfn) != entry)
> > + return true;
> > + }
> > + return false;
> > +}
> > +
> > +void kvm_arch_update_mem_attr(struct kvm *kvm, struct kvm_memory_slot *slot,
> > + unsigned int attr, gfn_t start, gfn_t end)
> > +{
> > +
> > + unsigned long lpage_start, lpage_end;
> > + unsigned long gfn, pages, mask;
> > + int level;
> > +
> > + WARN_ONCE(!(attr & (KVM_MEM_ATTR_PRIVATE | KVM_MEM_ATTR_SHARED)),
> > + "Unsupported mem attribute.\n");
> > +
> > + /*
> > + * The sequence matters here: we update the higher level basing on the
> > + * lower level's scanning result.
> > + */
> > + for (level = PG_LEVEL_2M; level <= KVM_MAX_HUGEPAGE_LEVEL; level++) {
> > + pages = KVM_PAGES_PER_HPAGE(level);
> > + mask = ~(pages - 1);
> > + lpage_start = max(start & mask, slot->base_gfn);
> > + lpage_end = (end - 1) & mask;
> > +
> > + /*
> > + * We only need to scan the head and tail page, for middle pages
> > + * we know they are not mixed.
> > + */
> > + linfo_update_mixed(lpage_info_slot(lpage_start, slot, level),
> > + mem_attr_is_mixed(kvm, slot, level, attr,
> > + lpage_start, start));
>
> Looks only query the lpage_start, start is not enough:
>
> A and B are private gfns from same lpage_start as below, A > B :
> lpage_start
> |---------A
> |----B
>
> Convert A to shared, this makes the upper 2M page to MIX.
> Convert B to shared, this also makes the upper 2M page to MIX.
> Convert B to private, this makes the upper 2M page to Non-MIX, but
> it's incorrect, due to A is shared.

In previous versions this is actually "lpage_start, lpage_start +
pages", e.g. covers the whole large page. While fixing another issue[*]
in v8 this was wrongly changed to "lpage_start, start", at that time I
made an assumption that "end > lpage_start + pages" so the remaining
scanning in the same large page is useless if we know what attribute we
will set, this is definitely not true though.

[*]
https://lore.kernel.org/linux-mm/20220930085914.GA2799703@chaop.bj.intel.com/

Thanks,
Chao
>
> Same to tail case.
>
> > +
> > + if (lpage_start == lpage_end)
> > + return;
> > +
> > + for (gfn = lpage_start + pages; gfn < lpage_end; gfn += pages)
> > + linfo_update_mixed(lpage_info_slot(gfn, slot, level),
> > + false);
> > +
> > + linfo_update_mixed(lpage_info_slot(lpage_end, slot, level),
> > + mem_attr_is_mixed(kvm, slot, level, attr,
> > + end, lpage_end + pages));
> > + }
> > +}
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 02ad31f46dd7..4276ca73bd7b 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12563,6 +12563,8 @@ static int kvm_alloc_memslot_metadata(struct kvm *kvm,
> > if ((slot->base_gfn + npages) & (KVM_PAGES_PER_HPAGE(level) - 1))
> > linfo[lpages - 1].disallow_lpage = 1;
> > ugfn = slot->userspace_addr >> PAGE_SHIFT;
> > + if (kvm_slot_can_be_private(slot))
> > + ugfn |= slot->restricted_offset >> PAGE_SHIFT;
> > /*
> > * If the gfn and userspace address are not aligned wrt each
> > * other, disable large page support for this slot.
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 4ce98fa0153c..6ce36065532c 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -2284,4 +2284,23 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr)
> > /* Max number of entries allowed for each kvm dirty ring */
> > #define KVM_DIRTY_RING_MAX_ENTRIES 65536
> >
> > +#ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM
> > +
> > +#define KVM_MEM_ATTR_SHARED 0x0001
> > +#define KVM_MEM_ATTR_PRIVATE 0x0002
> > +
> > +#ifdef __KVM_HAVE_ARCH_UPDATE_MEM_ATTR
> > +void kvm_arch_update_mem_attr(struct kvm *kvm, struct kvm_memory_slot *slot,
> > + unsigned int attr, gfn_t start, gfn_t end);
> > +#else
> > +static inline void kvm_arch_update_mem_attr(struct kvm *kvm,
> > + struct kvm_memory_slot *slot,
> > + unsigned int attr,
> > + gfn_t start, gfn_t end)
> > +{
> > +}
> > +#endif
> > +
> > +#endif /* CONFIG_KVM_GENERIC_PRIVATE_MEM */
> > +
> > #endif
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index fc3835826ace..13a37b4d9e97 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -939,7 +939,8 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
> >
> > #ifdef CONFIG_KVM_GENERIC_PRIVATE_MEM
> >
> > -static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end)
> > +static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end,
> > + unsigned int attr)
> > {
> > struct kvm_gfn_range gfn_range;
> > struct kvm_memory_slot *slot;
> > @@ -963,6 +964,7 @@ static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end)
> > gfn_range.slot = slot;
> >
> > r |= kvm_unmap_gfn_range(kvm, &gfn_range);
> > + kvm_arch_update_mem_attr(kvm, slot, attr, start, end);
> > }
> > }
> >
> > @@ -970,7 +972,6 @@ static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end)
> > kvm_flush_remote_tlbs(kvm);
> > }
> >
> > -#define KVM_MEM_ATTR_SHARED 0x0001
> > static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> > bool is_private)
> > {
> > @@ -979,6 +980,7 @@ static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> > void *entry;
> > int idx;
> > int r = 0;
> > + unsigned int attr;
> >
> > if (size == 0 || gpa + size < gpa)
> > return -EINVAL;
> > @@ -992,7 +994,13 @@ static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> > * Guest memory defaults to private, kvm->mem_attr_array only stores
> > * shared memory.
> > */
> > - entry = is_private ? NULL : xa_mk_value(KVM_MEM_ATTR_SHARED);
> > + if (is_private) {
> > + attr = KVM_MEM_ATTR_PRIVATE;
> > + entry = NULL;
> > + } else {
> > + attr = KVM_MEM_ATTR_SHARED;
> > + entry = xa_mk_value(KVM_MEM_ATTR_SHARED);
> > + }
> >
> > idx = srcu_read_lock(&kvm->srcu);
> > KVM_MMU_LOCK(kvm);
> > @@ -1005,7 +1013,7 @@ static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> > goto err;
> > }
> >
> > - kvm_unmap_mem_range(kvm, start, end);
> > + kvm_unmap_mem_range(kvm, start, end, attr);
> >
> > goto ret;
> > err:
> > --
> > 2.25.1
> >